Process for separating c5 diolefins



Oct. 21, 1952 MoDlFvElz 15 El-Q c. E. MORRELI. UAL

PROCESS FOR SEPARATING C5 DIOLEF'INS Filed ochzz, 194s F'lzAcTxoNALDlsrll-LATION CQLUMNS Eon-RACT l v E CQLUM N JZO FFI NATE Ex'rizACreDDuma IDOL/Av` N Sou/ENT ExTRAcTEo Charles E. morrcll Laef m. wachKme-@Lors b5 M Cltborrz eg Patented Oct. 21, r1952 PROCESS `FORSEPARATING C DIOLEFINS` Charles E. Morrell, Westfield, and Lester M.Welch, Madison, N. J., assignors to Standard Oil Development Company, acorporation of Delaware Application October `22, 1948, Serial No. 56,042

(Cl. .M2-39.5)

7 Claims.

This invention4 relates to a method for separating and concentrating C5diolens from their hydrocarbon mixtures containing other types ofhydrocarbons, such as present in a cracked petroleum naphtha fraction.More particularly, it is concerned with the recovery of purifieddiolens, such as soprene, cyclopentadiene and piperylene,

Hitherto, elcient recovery of the individual dioleiins, isoprene and itshigher boiling homologs, has involved a difficult fractionaldistillation problem of obtaining cuts that would contain the desireddiolei'lns in maximum amounts with the least amount of close-boilingcontaminants. For example, in obtaining an isoprene-rich C5 fractionfrom a cracked l,petroleum naphtha` by fractional distillation it isvery diil'icult to prevent the mono-olefin components trimethylethyleneand even cyclopentene from being present because of their similarvolatilities in the fraction taken. points of such contaminants in purestate, their volatilities become changed `or modified by the presence ofother components, and in subsequent stages of the separation process itis difficult to eliminate them.

Due to the dilculty of separating the hydrocarbons by fractionaldistillation, extractive distillation with various polar solvents hasbeen employed in subsequent stages, but the mentioned troublesomemono-olens tend to remain with the diolens even in the presence of thepolar solvents.

In accordance with the present invention, the concentrationand'purication of the C5 diolens is improved by subjecting the C5diolen-containing fractions to modiiied fractional distillations byadmixing relatively large amounts of parafnns of similar or lowervolatilities for depressing the Volatilities of the more troublesomemonoolefins, especially in a fractional distillation zone where theunsaturated hydrocarbons undergo rectification. There are certainadvantages in using, as the fractional distillation modifier, Csparaflins which distill together with the diolens being separated as adistillate, because these parailins tend to concentrate in therectification zone and then'ar'e' easily removedirom the diolefndistillate on subsequent extractive distillation using a polar solvent.There are certain Desp-ite the somewhat higher boiling other advantagesin using relatively higher boiling `C54- parafns as modifiers, byintroducing them to an upper part of the rectification zone where theytend to increase the volatilities of the diolen, although they alsodilute the residual hydrocarbons.

The appropriate manner of selecting and utilizing the modifiers andtheir respective advantages will be further explained in discussingtheoperation of the invention with reference to the acu companyingdrawing, which is a schematic flow plan of apparatus for employing thevarious types of fractional distillation modifiers in conjunc tion withsubsequent extractive distillations.`

In the drawing, the distillation systemis illustrated for treatment ofan isoprene-rich C5 fra-ction to eliminate the troublesometrimethylethyli ene and cyclopentene, when separating and purlfyingsoprene, or for treatment of wider boiling fractions, e. g., a C5 toCmnaphtha fraction to do likewise, while at the same time providingforthe recovery of other puried diolefin components, such as, piperyleneand cyclopentadiene free of both the troublesome mono-olens,trimethylethylene and cyclopentene.

The isoprene-containing fraction of narrow or wide boiling range isintroduced from feed line l into an intermediate part of thedistillation column I0, which may be constructed and equipped withconventional means, such as bubble lplates and .reboilen Fractionatedvapors of isoprene being separated.v from close-boiling mono-oleilns,trimethylethylene and cyclopentene, pass upwardly through therectification zone in the upper part of column I0 from the feed plate.Isoprene-rich distillate may be removed as a side-stream from one of thetop plates, e. g., through line 3 or may be withdrawn by line 4 fromreceiver 5 of overhead product that has passed from the top of column l0by line 6 through a cooling condenser l.

The overhead product from column I0 need `be only partially condensed incondenser 1 for obtaining liquid reflux and the remainder of theoverhead product in vapor phase may then be passed by line 4 fromreceiver 5 and by line 3 to the extractive distillation column 20. If`gaseous C4 hydrocarbons are in the overhead product from column I0, thenit becomes desirable to condense the isoprene-containing C5 overheadprodthrough line I I.

uct and vent the C4 gases from the receiver 5 through pipe 2.

A portion of condensate is returned as reflux from receiver 5 by line II to the top plates of column I0. Another portion of liquid distillate,which contains a high proportion of C5 paraflins may be advantageouslyreturned by line I4 to column II] at or near the feed plate, from whichtheir vapors ascend to build up the concentration of C5 parafns on theupper plates. However, as a substantial amount of the C5 parafiins, orpentanes, are withdrawn together with isoprene and lower boilingpenten'es inthe distillate, it is necessary to add more pentanes asmodifier,

e. g. through line I4 in order to maintain thel required highconcentration` of paraffns for de-I l pressing the volatility oftheclose-boiling olefins,v

trimethylethylene and cyclopentene,.unless ak higher boiling paraflinmodifier is introduced at the upper part of column Iby line-I3. 'Y

It will be noted that there are at least three different ways in whichthe. paraiiinic volatility. modifiers having at least 5 carbon'atoms'per' molecule can be supplied to the rectification zone.-

of column I0, but the ecient use of each way is dependent on the type offeed processed .and particular paraffin mainly used as a modifier. Ifthe modifier mainly used is isopentane, the fractional distillation'column may be operated to make the overhead product contain principallyisopentane, and the isopentane condensate is advantageously refluxedto aplate near the feed pltae, or at least below the plate from which anisoDrene-rich" distillate side-stream is withdrawn by line 3 so that theisoprene Will not be excessively diluted by isopentane. If normalpentane is used mainly as a modifier, it` may be returned largely withreflux passed from receiver 5 Additional quantities of the pentanesemployed as modiers maybe admixed with the feed or be introduced by lineI4.

In using the less volatile paraffinic modifiers such vas hexanes orparaflinic naphtha or'parafnic kerosene hydrocarbons, theyareessentially introduced into the upper part of the column, e. g., byline I3, preferably below the side stream plates and are then withdrawnas diluent in the residual hydrocarbons remo-ved fromthe bottom ofcolumn Il) by line I5 for further processing.

In any event, the concentration of theCs or higher paraflinichydrocarbons is continuously maintained suiciently high in therectification Zone of columns Il) so that the volatility of thetroublesome olefins, trimethylethylene and' cyclopentene is depressed,and these olefins are thus retained in liquid residual bottoms while thefractional distillation is conducted so that the isoprene is distilledtogether with pentanes and lower boiling pentenes. Some of the pentanesand some of the low boiling pentenes also tend to remain in the liquidresidue vcontaining the trimethyl ethylene and cyclopentene as well asthe diolens higher boiling than isoprene from the feed, i. e.,-cyclopentadiene and piperylene, but less of the pentanes and normalpentenes remain in this residue than if the less volatile Cs-I- parafnsare used as modifiers.

The isoprene distillate or overhead vapor fraction from column I0 is ofsuitable composition for segregating specification quality isoprenetherefrom by extractive distillation using the common polar solventswithout the interference that would arise from the` presence oftrimethylethylene or of cyclopentene.

Extractive distillationwith a polar solvent is not novel per se but isimproved in its efficiency for separating the various C5 diolefins bythe treatment of this invention. The extractive distillation includesthe use of known types of operations and polar solvents, such as aqueousacetone, sulfolanes, furfural, or the like, which have selective highsolvency for diolens but which at the same time tend to extract thetroublesome mono-olens that are close-boiling to but somewhat higherboiling than the diolefn being extracted, if they are present with thediolefin, as is shown inthe following tabulated data:

TABLE I Volatzlzty relationships of C5 hydrocarbons Relative VolatilityK f Bolling ,Hydrocarbon Conies Point ponent Fraction 251g.s.1.g. with fF. Normal 1 Solvent 1 lBased on normal vaporpressures, uncorrected forabnormal deviations.

2 2 vols. solvent (92 Wt. percent acetone, 8 wt. percent water) per vol.hydrocarbon in liquid phase.

3 Cis and trans. average.

The figures in the third column of Table I show that while the other C5diolens are considerably less volatile than isoprene, n-pentane and thenear-boiling C5 olens have normal volatilities ranging from 0.88 to 1.06with respect to isoprene. Thus, while it is practical to separateisoprene from other C5 diolefins by ordinary fractional distillation,equipment of impractical dimensions would be required to separate theisoprene from these near boiling mono-olens. Consequently, it has beenfound necessary to use extractive distillation in the presence of polarsolvents (e. g. acetone or others) for increasing the relativevolatilities of the near-boiling monoolens to permit their removal fromthe isoprene. But even so, there has been the remaining difficulty thatthe higher boiling mono-olefins, trimethylethyleneand cyclopentene aredifficult to remove from the diolefins by extractive distillation, andthen cannot be separated from the diolefins by further fractionationafterwards. 4Trimethylethylene is the most troublesome contaminant withrespect to isoprene, and cyclopentene is most troublesome with respectto cyclo'- pentadiene andpiperylene. Moreover, even some cyclopentadienetends to remain with the lsoprene concentrate obtained on ordinarydistillationy and an extractive distillation in the presence gf. polarsolvents subsequent to ordinary distilla- The isoprene-rich cut.obtained by the modied fractional `distillation in column IIJ has thetrimethylethylene, Cyclopentene, cyclopentadiene, and piperylenesubstantially eliminated, so that when this isoprene-rich cut issubjected to extractive distillation with a polar solvent in column 20,the isoprene is readily separated from in high purity with goodrecovery.

The polar solvent, which is largely recycled lean solution from line I8and fresh make-up solvent from line I9 is introduced into the upper partof column so as to redux countercurrently to the hydrocarbon vapors fromthe' hydrocarbon feed introduced by line 8. The pcntenes and pentanes,of increased volatility relative to the isoprene pass overhead fromcolumn 20 by line 2l through cooling condenser 22, and condensatecollected in receiver 23. Some of the condensate is returned as refluxfrom the receiver 23 by line 24 and a remaining portion of the distilledpentenes and pentanes is withdrawn as raffinate from receiver 23 by line25.

It may be noted that raffinate from extraction column 2B makes a goodsource of the pentane modifying agent useful in column Il).l The pen-`tenes can be separated from the pentanes in the raffinate by anysuitable process, such as polymerization, to remove the olefins andconcentrate n tilized components of the solution are returned to column20 by line 28. In a lower part of vessel 2l, the isoprene is desorbedfrom the extract solution and the lsoprene vapors are removed as a sidestream through line 29, leaving a lean extract solution or polar solventbottoms for recycling by line I8 to an upper part of column 20. Aportionof the lean solution may be Withdrawn for clean-up through lineI'I.

The residual fraction containing trimethylethylene, cyclopentadiene,piperylenes, cyclopentene and higher boiling hydrocarbons in column IBis next most effectively treated for separating the desired diolens by afractional distillation that is again modified through the use of the`parafns which depress the volatilities .of the trif methylethyleneandcyclopentene so as to avoid complications in a following extractivedistillation of the diolens contained in this residual fraction.

Thus, when the residual fraction is passed from l the bottom of columnl0. by line I5 into the fractional distillation column supplied withmodifying parafn, cyclopentadiene and piperylene is distilled overheadwith any pentanes and lower boiling pentenes, ethylenes, butcyclopentene is made to remain in the residue discharged through lineI6. l

The diolefimrich vapors freed of cyclopentene pass overhead from column30 by line 3l through cooler 32 for total or partial condensation..ySome ofthe condensate from receiver 33 is refiuxed to column 3D by line34, and a. remaining portion of the distillate or overhead vapor ispassed along by line 35 to a subsequent extractive distillation unit 40.

- The distillation modifying paraihns can be supplied to column 30, asthey are supplied to column Ill, e. g., low boiling `Caparafns by line3.6, high boiling Cs-I- parafnns by line 3T.

As in the extractive dstillationcarried out on the isoprene distillatein column 20, extractive distillation with a polar solvent is performedin column 4I!` to remove pentanes and the pentenes lower boiling thanthe diolens overhead `from column by line 4I through condenser 42 toincluding trimethyl- 6 receiver-43, whence a portion 'of' condensate "isrefiuxed by line 44 to an upper part of column 40 and a remainingportion is Withdrawn. asrainate byline 45. i The polar solvent, recycledby line `46 and any fresh make-up solvent from line 4l, enters a toppart of column 40 toy pass `countercurrently to vapors from thehydrocarbons entering onto a feed plate from line 35. Extract solutionis withdrawn as bottoms by line 48 to the upper part of the strippingand desorption vessel 49, Where some volatile hydrocarbons are releasedand returned with some diolen and solvent vapors into the bottom part ofcolumn 40 by line 50. In a desorption zone, at the lower part of vessel49, the piperylene with some cyclopentadiene are desorbed and removed asa vapor side-stream ythrough line 5I for recovery. The lean solution orsolvent bottoms is recycled by line 46 except for a portion removed byline 52 for clean-up an reconcentrating. l A

With the general operability of the process for separatingtrimethylethylene from isoprene and ofseparating cyclopentene from thehigher C5 diolens demonstrated, engineering of the proc'- ess is`carried out to suit the needs of the particular feed materialsavailable, each of the distillation columns and of the stripping anddesorption vessels being made of adequate size with a suitable number ofplates. f 4 f l `For example, the initial separation of an iso-'prene-rich distillate free of trimethylethylene is carried out incolumn Ii) by having a cracked 1naphtha or C5 fraction enter near themiddle of the column I0 having a top temperature of about 14.0 F. at 25p. s. i. g. and a higher bottoms tem4` paraffin content is increasedy toabove 40 mol`% in liquid on upper plates of the column, the amount oftrimethylethylene collected' with the isoprene distillate is negligible.The residual bottoms fraction continuously Withdrawn contains the higherboiling C5 diolens, low and high boiling parafiins, some of the, normalpentenes, the trimethylethylene, cyclopentene, cyclopentane, and any ofthe Ce and higher hydrocarbons which were present in the initial feed.

Carrying out the modified fractional. distillation of the residualbottoms fraction from column with the distillate is negligible althoughthe ,tri--` methylethylene distills over with these diolens. Thecyclopentene remains in' the residue with higher boiling paraflins and(16+ hydrocarbons.

The following data in Table II shows how the presence of cyclopentenewould make theseparation of the diolens substantially impossible in theextractive'distillation of the close-boilingdiclaims, cyclopentadieneand pperylene, considering that a relative volatility of the order of1.2 to v1.3 for the component being distilled vWith respect to thecomponent to be retained in the residue is necessary.

1 At atmospheric pressure with no polar solvent. 2 At 25 p. s. i. g., 2parts solvent to one of, hydrocarbons. 3 0.94 with no polymerization,0.64 for polymers.

Thus it can be seen that by elimination of cyclopentene in the priorfractional distillation modified through the addition of parafns then inthe extractive distillation the difficulty of removing cyclopentene isavoided and the other olefins and parains are increased in 4volatilitysuiciently by the polar solvent to eliminate them as part of theraffinate.

The desired C5 diolens,useful in synthetic rubber, resin, and othermanufacturing processes if they are of adequately high purity, areformed in relatively small quantities by high temperature cracking ofpetroleum hydrocarbon oils, and they appear in distillate productscontaining monoolens, parains and large amounts of aromatics. Thedebutanized aromatic distillates often contain about Wt. of C5hydrocarbons with Cs C10 hydrocarbons, the C5 hydrocarbons and theirtypical proportions being indicated in Table I.

In processing the Cia-C10 cracked distillates to concentrate and purifythe C5 diolens, a narrow C5 cut may rst be obtained, butit is moreeconomical to eliminate any first step of superfractionation Whichrequires large fractionation equipment for the initial large amount ofmaterial.

With the process of the present invention the Cs cut may first beobtained or the debutanized naphtha containing Cia-Cin hydrocarbons canbe subjected to the major steps already outlined and summarized asfollows: (1) Modified fractional distillation of the C5 cut or a naphthafraction containing the C5 hydrocarbons using C5 or higher paraiiins todepress the volatilities of trimethylethylene and cyclopentene to obtainan isoprene-rich distillate `free of these troublesome mono-olefins.

(2) Extractive distillation of the isoprene-rich distillate from themodified fractional distillation to recover isoprene free oftrimethylethylene, which is kept in the bottoms of step (l) withpiperylenes, cyclopentadiene and cyclopentene.

(3) Modified fractional distillation of a C5 or debutanizedv naphthafraction freed of isoprene but `containing cyclopentadiene andpiperylenes contaminated by cyclopentene as in the bottoms of step (1),using C5 or higher paraiins to depress the volatility of the.cyclopentene relative to the diolei'lns.

(4) Extractive distillation of the cyclopentadiene andpiperylenes freeof cyclopentene .to remove other close-boiling parafns and monoolens,including trimethylethylene.

The separation of isoprene from its most troublesome contaminant,trimethylethylene, in the first step by the modified fractional distillation .has been demonstrated to be due to changes in relativevolatilities responsive to increased concentrations of the paraiins asshown in the following summarized data:

TABLE' III Relative Volatilities Hydrocarbon Component Modied K NormalBy Bytlso' 1 nPentane pen ane Pentene-i 1. 14 1. o4 o. 98 isoprene1.00 1. 00 1, 00 Trxmethylethylene 0. 88 0. 82 0. 81

The above listed relative volatilities are based on experimental data ofthe volatilities of the components to n-pentane and isopentane,respectively, as the liquid content of the paraifin'is increased toabove 40 mole in twocomponent systems of the unsaturated hydrocarbonsand the parains, the normal relative volatility of isopentane being ofthe order of 1.26 with respect to isoprene, which is given the basevalue of 1.00 for its volatility.

In a three-component system containing isopentane, isoprene, andtrimethylethylene, it'was found experimentally that the volatility ofthe trimethylethylene with respect to isoprene was lowered still furtherto 0.77. Thus, with the relative volatility isoprene totrimethylethylene at 1.3, (1 /0.77) the separation of isoprene is madepossi'- ble with practical fractionating equipment.

In the second major step, the volatilities of mono-olefins are reducedrelative to those of the diolens by increased amounts of a paraflin formodifying the fractional distillation. Again using the volatility ofisoprene as a base (1.00), the summarized data given in the followingtable shows how the mono-olefin cyclopentene, most troublesome withrespect to piperylene and cyclo- 'pentadiene is given a sufficientlylowered relative volatility so that it is readily kept in the residue,wh11e the trimethylethylene of similarly depressed in volatility willvolatilize with these diolefins:

In Table IV the relative volatility values are based on calculationsfrom observed volatilities of binary systems containing one of thelisted olennic components and the paraiiin in concentrations increasedto above 40 mole The figures given shows the trend by which the diolennsare made .more volatile and the mono-olefins are .made relative lessvolatile by the parafn. For

example, if no paraffin is present in a system of trlmethylethylene andcyclopentadiene, the tripentene.

:3.9 methylethyleneis aboutyi times more volatile than thecyclopentadiene, whereas in the pres- ,ence of the added parain thecyclopentadiene is about as volatile as the trimefthylethylenaand atVthe same time, more volatile than the cyclo- ,l In the rnore 'complexmixtures Vthe modifying effect of the paraflinis'indicatedto'be .levenmore accentuated for assisting thesepara- )tion of theCs diolefins fromcyclopentene.'

Although trimethylethylene clistills overtwith the cyclopentadiene andpiperylene in themodiclopentene in the fourth major step of extractivelydistilling the distillate containing cyclopentadiene and piperylene witha polar solvent, the relative volatility of the trimethylethylene issufliiciently high with respect to these diolelns so that thetrimethylethylene is distilled over as part of the rafiiiiate.

A large variety of alternative polar solvents may be used in each of theextractive distillation steps, although aqueous acetone is chosen as apreferred solvent. The other polar solvents, such as, glycol diformate,acetonitrile, furfural, sulfolanes, etc. Well known in the art aresimilarly eiective in lowering the volatilities `of the dioleiinsrelative to close-boiling parafiins and mono-oleiins but neverthelessare difficult to use for separating trimethylethylene from isoprene andfor separating cyclopentene from cyclopentadiene and from piperylene.

It `will be understood by those skilled in the art that regardless ofwhat kind of extractive distillation or iinishing process is employed,the fractional distillation modified by the addition of paraflinhydrocarbons having at least carbon atoms, eg. pentanes and their higherhomologs, as provided herein, is a useful method for segregating a C5dioleiin from a close higher boiling C5 mono-olefin. This modificationin the fractional distillation of 'mixtures containing C5 diolefins andmono-olens can be varied to suit the needs for the mixtures treated byvarying the amounts and kind of paraffin employed as modifier althoughfor typical cracked petroleum fractions which contain fairly smallamounts of paraffins indigenously, the added amount of this modifier issuch as to give at least a concentration of 40 mole oi parai'ns inliquids on the rectification zone plates.

What is claimed is:

1. In the separation and` purification of isoprene from a mixturecontaining lower boiling pentenes and trimethylethylene, theimproveprises fractionally distilling the isoprene with F' the normalpentene and pentanes from said fraction in the presence of addedparaflin hydrocarbon of 5 carbon `atoms per molecule to reduce thevolatility of the trimethylethylene relative to the volatility of theisoprene, adding a lOM lsufficient amount of` said paraiiin hydrocarbon.tomaintain a proportionfof at least `40 mole ,ofparafns having at least5 carbonatoms per molecule in liquid `phase portions of said fractionundergoing fractional distillation, separating a distillation cutlcontaining the isoprene, normal pentene, and pentanes volatilized fromsaid liquid portions ofthe fraction undergoing fractional distillation,and recovering the triniethylethylene in 'a residual liquid portion ofsaid fraction in which remains 'a portion of said added parai'linhydrocarbon.

3. The method of claim 2, in which isopentane is the added paraffinhydrocarbon.

4. Ihe method of claim 2 in which the added paraffin hydrocarbon isisopentane, and in which isopentane is cooled as an overhead distillate,then is returned to the fractional distillation.

5. In the separation and purification of isoprene from a liquidmixturecontaining pentene-l, trimethylethylene, cyclopentadiene, andcyclopentene, the improvement which comprises fractionally distillingthe isoprene with the i pentene-1 from the liquid mixture in afractional distillation zone, while giving liquids in said fractionaldistillation zone a content of at least 40 mole of parafns having 5carbon atoms per molecule by addition of said parafns, Withdrawing fromsaid fractional distillation zone isoprene mixed with pentene-1 andwithdrawing from said fractional distillation Zone a residual portion ofsaid liquids containing the trimethylethylene and cyclopentadiene mixedwith at least a portion of said added paraiins.

6. In the separation and puriiication of isoprene and piperyleneinitially mixed with C5 mono-olens and C5 paraffins, the improvementwhich comprises separating an isoprene-rich cut containing isoprene withC5 paramns and lower boiling C5 mono-oleiins from said mixture byfractional distillation in the presence of added paraffins having 5carbon atoms per molecule, said added paraflins being admixed to givethe liquid mixture a content of at least 40 mole of the parafns duringthe fractional distillation so that trimethylethylene remains in aresidual portion of the liquid mixture undergoing fractionaldistillation, then fractionally distilling a piperylene-rich cut fromsaid residual portion containing trimethylethylene, cyclopentadiene, andcyclopentene in the presence of added C5 parafns forming at least 40mole of liquids from which the piperylene is being distilled so thatcyclopentene remains in a resulting bottoms portion, fractionallydistilling trimethylethylene from said piperylene-rich cut by extractivedistillation in the presence of a polar solvent and fractionallydistilling piperylene from the polar solvent extract of thepiperylene-rich cut to obtain a final piperylene distillate freed ofisoprene and trimethylethylene and of cyclopentene.

'7. In the separation and puriiication of piperylene from a mixturecontaining trimethylethylene and cyclopentene, the improvement whichcomprises adding to said mixture a C5 paraffin hydrocarbon in an amountto make C5 paraiiin hydrocarbons form at least 40 mole of the resultingliquid mixture, and fractionally distilling the piperylene with thetrimethylethylene and C5 parain so as to leave the cyclopentene in aresidual portion of the mixture, thereafter fiactionally distilling thetrimethylethylene with C5 paraffin from the piperylene by extractivedistillation in the presence of a polar solvent for piper- 1l ylene,then dstlln'g pperylene from the resulting polar solvent extract of thepperylene to obtain a piperylene distillate freed of trmethylethyleneand of cyclopentene.

CHARLES E. MORR'ELL.

LESTER M.

REFERENCES CITED The followingv references are 0f record in the flle ofthis patent:

12 STATES PATENTS Number Name Date Gylol --.1- July 12, 1938 PattersonOct. 31', 194,4 Evans Apr. 3, 1945 Patterson v Sept. 24, 1946 Pattersonet a1. v Sept. 2', 1947 McKinnis 1 Dec. 23, 1947 Morris et al Jan. 1.3,1948 "rookie et; a1 1-- Mar. 22, 1949

1. IN THE SEPARATION AND PURIFICATION OF ISOPRENE FROM A MIXTURECONTAINING LOWER BOILING PENTENES AND TRIMETHYLETHYLENE, THEIMPROVEMENT, WHICH COMPRISES FRACTIONALLY DISTILLING AN ISOPRENE-RICHCUT CONTAINING THE LOWER BOILING PENTENES FROM THE MIXTURE WITH ADMIXEDISOPENTANE SO AS TO LEAVE THE TRIMETHYLETHYLENE IN A RESIDUAL PORTION OFTHE MIXTURE, SAID ISOPENTANE BEING ADMIXED IN A PROPORTION OF AT LEAST40 MOLE % OF LIQUID UNDERGOING FRACTIONAL DISTILLATION, AND THEREAFTERFRACTIONALLY DISTILLING THE LOWER BOILING PENTENES FROM THEISOPRENE-RICH CUT IN THE PRESENCE OF A POLAR SOLVENT WHICH EXTRACTS THEISOPRENE.